Experimental investigation of fatigue strength of out-of-plane gusset welded joints under variable amplitude plate bending loading in long life region

This study investigated fatigue strength of out-of-plane gusset welded joints under variable amplitude plate bending loading in a long life region. First, constant amplitude stress fatigue tests using a plate bending fatigue test machine were previously carried out under R = 0.0, around 0.5, and - 1. Its fatigue limit (constant amplitude fatigue limit (CAFL)) was assumed from the minimum stress ranges in which fatigue failure occurred and was approximately 51, 40, and 45 MPa, which tended to depend on the stress ratio. Thereafter, a loading system for the machine was originally developed to handle variable amplitude stress. The system's accuracy was verified by a ratio of output versus input data and improved up to around 90% by slightly modifying applied loading conditions. Then, variable amplitude stress fatigue tests were carried out using the developed system under several magnitudes of variable stress ranges determined based on the CAFL. From the results, fatigue crack initiation and propagation occurred when any of the stress components exceeds the CAFL, despite the equivalent stress range being lower than it, and damage accumulation could be evaluated using the modified Miner's rule. In contrast, when none of the stress components exceeds the CAFL, fatigue crack initiation was not observed even after 500 million cycles.